1. Field of the Invention
The present invention relates to a photomultiplier tube which can operate even in a ferromagnetic field and, more particularly, to a photomultiplier tube used in the field of high-energy physics.
2. Related Background Art
A conventional technique in such a field is disclosed in Japanese Patent Publication No. 3-81257. The photomultiplier tube described in this reference is used in a ferromagnetic field. This photomultiplier tube converts light into photoelectrons by using a photocathode (photosensitive surface) formed on the transparent light-receiving faceplate of a sealed vessel. Photoelectrons emitted from the photosensitive surface strike a dynode in the form of a circular truncated cone first, and then are captured by an anode formed from a metal grating having a transmittance of 80 to 90%. As a consequence, predetermined photodetection can be performed even in a ferromagnetic field.
The above conventional photomultiplier tube, however, suffers the following problem. The anode used by this photomultiplier tube must be worked into the form of a circular truncated cone in accordance with the shape of the dynode in the form of a circular truncated cone. It is very difficult to work an anode in such a shape by using a very thin mesh plate. The formation of an anode in the form of a circular truncated cone will increase the process cost. If a thicker mesh plate is used to facilitate working an anode, photoelectrons emitted from the photosensitive surface have the difficulty in passing through the anode. As a result, photoelectrons are captured by the anode before they strike the dynode, resulting in a deterioration in gain.
Note that photomultiplier tubes for use in a ferromagnetic field are disclosed in, for example, Japanese Patent Laid-Open Nos. 4-345741, 5-82076, and 9-45275.
The present invention has been made to solve the above problems, and has as its object to provide a photomultiplier tube designed to decrease a process cost while improving gain characteristics.
A photomultiplier tube including a photosensitive surface for emitting photoelectrons in accordance with light incident on a light-receiving faceplate, a dynode for emitting secondary electrons upon receiving the photoelectrons emitted from the photosensitive surface, and a mesh-like anode for collecting the secondary electrons is characterized in that the anode is disposed to be parallel to the photosensitive surface, and the dynode has a secondary electron emission surface tilted with respect to the anode.
In this photomultiplier tube, light incident on a light-receiving faceplate is converted into photoelectrons by a photosensitive surface, and the photoelectrons strike a dynode to emit many secondary electrons. The secondary electrons are then collected by a mesh-like anode. Since the anode is disposed to be parallel to the photosensitive surface, the photoelectrons emerging from the photosensitive surface can easily pass through a mesh portion, and many photoelectrons can be made to strike the dynode. As the number of photoelectrons incident on the dynode increases, the number of secondary electrons from the dynode increases. This improves the gain characteristics of the photomultiplier tube. In addition, since the anode is formed to have a flat shape conforming to the shape of the photosensitive surface, the mesh-like anode can be easily molded. A secondary electron emission surface of the dynode is tilted with respect to the anode, photoelectrons having passed through the anode obliquely strike the secondary electron emission surface of the dynode. As a consequence, the number of secondary electrons emitted can be increased. This also improves the gain characteristics of the photomultiplier tube.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.